Phosphogypsum (PG, CaSO4·2H2O) is a waste by-product produced during the wet manufacturing phosphoric acid process. Phosphoric acid is manufactured by processing phosphate rock that contains relatively high concentrations of naturally occurring radioactive impurities which makes PG a radioactive material. PG also contains certain trace metals including As, Ba, Cd, Cr and Pb and radioactive contaminants such as Radium226 and Uranium238 that pose potential hazard to human health and the environment. According to USEPA, the current allowable disposal method for PG is stack piling.

This research was focused on stabilizing PG with Portland Type II cement and Class C fly ash for use in marine environments. The specific objectives included the development of a PG briquette composition that under submerged conditions: 1) maintained physical integrity, 2) minimized dissolution of Ca2+, SO42-, Ra226 and toxic metal concentrations in saltwater, 3) possessed appropriate engineering properties for its potential as a fill material and 4) was economically feasible to fabricate.

The 73%:25%:02%, 67%:30%:03%, 63%:35%:02% and 62%:35%:03% PG:Class C fly ash:Portland Type II cement composites showed promising results with no signs of degradation after 12-months of natural saltwater submergence. The leaching behavior of all the PG composites was found to be similar and the effective calcium and sulfate diffusion coefficients ranged 1.36-8.04x10-13 m2·s-1 and 2.96-7.20x10-13 m2·s-1, respectively. The Ra226 concentrations leached were observed to be safe, and ranged well below the current EPA regulation for the amount of Ra226 concentration in safe drinking water (5 pCi·L-1). The metal concentrations in the TCLP leachate were well below the USEPA toxicity characteristics limits. The engineering properties test results indicated that the composite material could be classified as well-graded gravel or well-graded sand with little or no fines. The USCS classification would also qualify the PG briquettes as a potential fill material in embankment construction having excellent workability characteristics.

The 73%:25%:02% PG:Class C fly ash:Portland Type II cement composite demonstrated the lowest production cost ($10.62 per ton for the year 2001 in Tampa, Florida) among the best four selected combinations.